Decanter centrifuge

ABSTRACT

The decanter centrifuge (1) comprises a conveyor screw (5) constructed as a hollow body with a lower average density than the lighter liquid phase of the surrounding slurry. The conveyor (5) thus flows in said liquid phase which is utilized in that the one traditional radial bearing of the conveyor is replaced by a radial support bearing (22) which only during starting and stopping of the centrifuge cooperates with the conveyor (5) in order to prevent it from contacting the bowl (2). In operation, the conveyor is thus only supported in the radial direction at the one end, entailing that the flexural rigidity of the conveyor becomes less important to the maximum allowable number of revolutions of the centrifuge which may then be increased for a given centrifuge, thereby obtaining an improved separating effect.

BACKGROUND OF THE INVENTION

This invention relates to a decanter centrifuge comprising an axiallysymmetrical bowl and a conveyor screw journalled therein.

Such a decanter centrifuge is employed for separating a slurry suppliedto the interior of the bowl into a solids phase and one or more liquidphases. This is obtained by rotating the entire centrifuge at a highnumber of revolutions and rotating at the same time the conveyor at alow number of revolutions relative to the bowl.

The separating effect of the centrifuge and its capacity or throughputdepend, on one hand, on the magnitude of the field of gravitationgenerated by the centrifugal force in the separating space of the bowl,i.e. on the number of revolutions and the inner diameter of the bowland, on the other hand, on the length of the separating space.

A factor of decisive importance for the maximum allowable number ofrevolutions is the flexural rigidity of the conveyor radially supportedat both ends of the bowl because the flexural rigidity determines thecritical number of revolutions of the conveyor.

This fact has hitherto implied that the λ-value of a givencentrifuge--the λ-value being defined as the ratio between the lengthand the inner diameter of the separating space--has not in practiceexceeded values of about 5.

In cases where a large field of gravitation and a large capacity havebeen required, the resulting centrifuges have been excessively large andexpensive. This relates to the fact that a straight geometricallyenlargement of a given decanter centrifuge has caused the costs ofmanufacture to increase by the cube of the scale ratio, while thecapacity only increases by the square of the scale ratio.

In view of the fact that, inversely, a capacity increase proportional tothe extension is obtained simply by increasing the length of thecentrifuge--without a corresponding rise in price--it is obvious to aimat producing decanters having λ-values exceeding said approx. 5.

SUMMARY OF THE INVENTION

This is now made possible by a decanter centrifuge according to theinvention which differs from the prior art centrifuges in that theaverage density of the conveyor, i.e. the ratio between its total weightand the volume it displaces, is smaller than the density of the lighterliquid phase of the actual slurry, and in that at least one end of theconveyor in operation is unsupported in the radial direction in relationto the bowl.

The invention will now be described in more detail with reference to thedrawings in which

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematical longitudinal section through a decantercentrifuge according to the invention, in operation, and

FIG. 2 is a section as FIG. 1, but through a second embodiment of acentrifuge according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The decanter centrifuge 1 illustrated in FIG. 1 consists by and large ofa horizontal, axially symmetrical bowl 2 with a cylindrical section 3and a conical section 4. The bowl 2 includes an elongated conveyor screw5 with a central body portion 6 and surrounded by a continuous screwflight 7. The bowl is at its ends rotatably supported in bearings 8 and9, respectively, and is driven via a gear 10--for example an epicyclicgear--ensuring in a known manner that bowl 2 and conveyor 5 in operationrotate relative to each other.

The suspension to be separated is supplied through an inlet 11 in theform of an inlet tube 12 and extending coaxially with the axis ofrotation of the centrifuge through a central passage 13 provided inconveyor body 6. The tube 12 ends in a transverse, radial passage 14discharging into the separating space 15 of the centrifuge. The liquidlevel in operation is illustrated in dotted lines 16 and solid lines 17.After separation in space 15 solids are discharged through apertures 18while liquid is discharged through an annular outlet 19.

Conveyor 5 is by and large constructed as a hollow body with closedcavities 20 and 21 and is so dimensioned that the average density of theconveyor as a whole is smaller than the density of the ligter liquidphase of the actual suspension. In this context the average density ofthe conveyor is defined as the ratio between the total weight of theconveyor and the volume it displaces.

In other words, the conveyor is able to flow in said light liquid phasewhich in operation constitutes the innermost portion of the "liquidtube" generated by centrifugal forces and positioned along the internalsurface of the bowl wall.

By adjusting the liquid level in separating space 15--i.e. the wallthickness of the liquid tube--so that the outer surface of body portion6 is substantially covered, it is obtained that the conveyor body may beregarded as approximately fully submerged and therefore in possession ofa certain positive buoyancy. The result is that the conveyor willconstantly seek towards the surface which in operation will mean towardsthe axis of rotation. The conveyor is in other words self-centering.Said effect has in practice proven to be independent of the degree ofoverlapping between the outer diameter of the body portion and the innerdiameter of the liquid tube, provided a certain overlapping exists.

This floating and self-centering ability of the conveyor in operation isin the embodiment illustrated in FIG. 1 utilized for omitting one of thetraditional radial bearings of the conveyor (in casu at the large end).

Instead there is, as illustrated, provided a radial support bearing 22the radial external surface of which is adapted to cooperateperiodically with the radial internal surface of a circular collar 23 onthe conveyor body. There is a certain radial clearance between the outerdiameter of the supporting bearing and the inner diameter of the collar.Consequently, the bearing and the collar will only cooperate when theradial oscillation of the conveyor from the axis of rotation at thelarge end exceeds said clearance. This will for instance be the casewhen the centrifuge does not rotate or rotates at such a low number ofrevolutions that the liquid tube cannot be maintained and thus has nosupporting ability. The clearance is so dimensioned that the conveyor inno circumstances can contact the bowl wall.

At the small end the conveyor is journalled in an ordinary way by meansof a traditional radial bearing 24. Said bearing may possibly be aspheric bearing in order to allow the slight angular movements of theconveyor which is a consequence of its radial deflection at the largeend. A second possibility of allowing said angular movement is to makeuse of a sort of a diaphragm coupling between the drive shaft of theconveyor at the small end and the conveyor body proper. Moreover, theconveyor is, in a manner not shown, controlled in the axial direction.

As mentioned in the introduction it is in current centrifuges theconveyor--or rather its flexural rigidity--which in practice determinesthe maximum allowable number of revolutions of the centrifuge.

This is due to the fact that the conveyor is constantly supported atboth ends in the radial direction. By suspending, in operation, theradial support of the one end of the conveyor, as described above, thedecisive influence of the flexural rigidity is reduced, meaning that theλ-ratio of the centrifuge as well as its maximum allowable number ofrevolutions can be increased.

The diminished demands on strength makes it possible to obtain theweight reduction necessary for constructing the conveyor with an averagedensity not exceeding the density of the lighter liquid phase, interalia because the conveyor body may now be made from sheet material. Asan example of the size of the weight reduction obtained it can bementioned that the conveyor of a given traditionally constructedcentrifuge has a weight of about 400 kg, while the conveyor of acorresponding centrifuge according to the invention can be manufacturedwith a weight of only about 100 kg.

FIG. 2 illustrates a second embodiment of a centrifuge according to theinvention, namely a so-called disc decanter 25, i.e. a centrifugeincluding a pile of discs as well as a conveyor screw.

Centrifuge 25 comprises a double-conical bowl 26 at the one end of whicha pile of discs 27 is arranged which rotates with the bowl and at theother end of which a conveyor screw 28 is mounted which in operationrotates relative to bowl 26. The actual slurry is supplied through aninlet 29 and the separated phases are discharged from the bowl throughapertures 30 and outlets 35 and 36, respectively, at the left-hand endof bowl 26 in FIG. 2. The liquid level in operation is designated by 31.The mode of operation of such a centrifuge is known and will thereforenot be explained in detail here.

As above, conveyor 28 is constructed as a hollow body the radialjournalling of which at the one end is effected by means of aperiodically active, radial support bearing 32 and an annular collar 33.In this case too, the other end of the conveyor is journalled by meansof an ordinary radial bearing 34 and the conveyor is likewise controlledin the axial direction in relation to the drum.

The radial support bearing is in both specified embodiments arranged atthe "large end" of the conveyor, this being advantageous because theconveyor has there a comparatively greater buoyancy than at the smallend. This relates to the fact that the body portion is conically taperedat the small end and thus accommodates less buoyancy promoting cavityvolume per unit of length than is the case at the large end. Moreover,the conveyor is not fully submerged at the small end. There is, however,nothing to prevent the support bearing from being positioned in otherembodiments at the small end, and it is also possible to provide theconveyor with support bearings at both ends.

I claim:
 1. A decanter centrifuge for separating a slurry into a heavierphase and a lighter phase, which comprises:an axially symmetrical bowl;and a screw conveyor, rotatably located in the bowl, having a body withat least one flight, wherein a specific density of the screw conveyor islower than a specific density of the lighter liquid phase of the slurryand, in operation, at least one end of the screw conveyor is radiallyunsupported.
 2. A decanter centrifuge as in claim 1, further comprisingmeans, disposed on the centrifuge, for radially supporting one end ofthe screw conveyor.
 3. A decanter centrifuge as in claim 1, furthercomprising means, arranged at the at least one unsupported end of thescrew conveyor, for preventing contact between the at least one flightof the screw conveyor and the bowl during starting and stopping of thecentrifuge.
 4. A decanter centrifuge for separating a slurry into aheavier phase and a lighter phase, which comprises:an axiallysymmetrical bowl; and a screw conveyor, rotatably located in the bowl,having a body with flights, wherein a specific density of the screwconveyor is lower than a specific density of the lighter liquid phase ofthe slurry, and, in operation, at least one end of the screw conveyor isradially unsupported, and whereby, when in operation, the at least oneunsupported end of the screw conveyor is centered in relation to thebowl by buoyancy forces acting on the screw conveyor.
 5. A decantercentrifuge as in claim 4, further comprising a bearing, disposed on thecentrifuge, for radially supporting one end of the screen conveyor.
 6. Adecanter centrifuge as in claim 4, further comprising a limiting device,arranged at the at least one unsupported end of the screw conveyor, forpreventing contact between the flights of the screw conveyor and thebowl during starting and stopping of the centrifuge.